1. Field of the Invention
The present invention relates to a call control method for handoff guarantee in a mobile terminal, in particular, for minimizing handoff failure rate so as to continuously guarantee the same quality of service during handoff of an existing call from the current cell into a new cell in a high-speed radio communication network which is constituted of micro-cells to support multimedia traffic.
2. Description of the Related Art
At present, a mobile communication network structure is composed of macro-cell units with application radii of several Km to provide a communication service mainly based upon audio and short messages. However, high transmission capacity is required to the mobile communication network by the following reasons: the number of mobile terminals increases, the amount of radio bandwidth is restricted, and it is necessary to support a multimedia application requiring a wide band service. In order to provide the foregoing, a future mobile communication network structure will be embodied as a micro-cell or pico-cell structure having an application radius of several hundred meters.
In the mobile communication network structure having such micro-cells or pico-cells, downsizing cells will reduce the application area of each cell while probably accompanying high handoff rate and incurring rapid variation to the traffic state of the network. This may barely guarantee the service quality about multimedia traffic.
Overload may occur in a specific cell due to movement of terminals, and thus adequate control about a new call should be carried out in preparation for a situation that a terminal moves into the overloaded cell. Otherwise, it is barely guaranteed that network resources allocated while initially setting the new call is identically used in the new cell.
This breaks the handoff which occurs as the existing call moves into the new cell fails, thereby disconnecting the call. Minimization of handoff failure rate is required so that the call can be continuously served without disconnection while the terminal moves. The handoff failure rate which does not exist in a wire communication network is a new and important parameter of system performance in the mobile communication network.
A call control method for reducing the handoff failure rate can be mainly divided into three schemes:
First, a handoff request queuing for allowing handoff, which queues a handoff call when a bandwidth is unavailable in a destination cell during handoff and allocates the bandwidth to the handoff call prior to a new call if there is any available bandwidth by using a time delay that a mobile terminal can allow within a handoff area overlapped between cells;
Second, an exclusive bandwidth reservation for partially allocating the bandwidth of a cell to previously reserve the bandwidth in preparation for handoff, in which the bandwidth is allocated by requesting reservations from all adjacent cells or part of cells having a high destination probability of a mobile terminal whenever a new call is initialized, and dynamically adjusted according to the communication network state in order to effectively use the radio bandwidth; and
Third, a restrictive approval scheme of a new call in preparation for handoff which collects state information from the current cell where a mobile terminal is located and surrounding cells adjacent to the current cell and approves connection of the new call based upon handoff provability into the adjacent surrounding cells according to the variation of future time depending on correct information about a movement pattern of the mobile terminal.
Therefore, supporting a multimedia service in the radio communication network has a number of sophisticated problems due to characteristics such as the mobility of the mobile terminal, restricted bandwidth and high error rate, which may not occur in the conventional wire communication network.
First, the mobile terminal can move to any place while the call connection is set so that handoff occurs during cell switching in the radio area due to the movement of the mobile terminal. The handoff should be so carried out that the existing connection may be not cut, and the service quality requested in the connection should be continuously guaranteed. Further, re-routing of the existing connection and change of a call connection path are accompanied, and control information for controlling the same consumes the insufficient radio communication resources. Therefore, it is required a method for minimizing the amount of control information necessary for the handoff while rapidly carrying out the handoff.
Second, when a call is set in the cell where the mobile terminal is currently located, the service quality is guaranteed by the communication network as long as the cell is not changed. However, when mobile terminals concentrically accumulate into any cell, the available bandwidth in the cell is temporarily insufficient for accepting the requests of the mobile terminals. Therefore, the current service quality in use may be degraded or the call may be disconnected. Such a phenomenon should be prevented from occurring while the call is set even though the mobile terminal is moving.
Third, the bandwidth of the radio communication network is considerably restricted compared to the wire communication network. The bandwidth is highly valuable resources in the radio communication network, thereby requiring a method for effectively using the same.